This invention relates to polymer bound antioxidant stabilizer compositions wherein the stabilizers are attached to anhydride polymers or copolymers by amic acid or imide formation. This invention also relates to the use of these polymer bound stabilizers to stabilize polymers or polymer blends against thermooxidative degradation.
When plastic materials are exposed to a high temperature environment either in processing or in final applications, degradation as evidenced by discoloration, cracking, and loss of mechanical properties generally occurs. To help overcome these effects a great number of heat stabilizer additives are commercially available. While most of these additives initially perform well in the plastic compositions, they tend to be removed over a period of time by vaporization, blooming (migration to the surface where they can rub off), thermal degradation, or the action of various solvents (extraction). Thus, removal of stabilizers leads to a shortened useful life of a polymer composition subjected to elevated temperatures.
Antioxidant stabilizers with low volatility are required for polymers that are extruded at high temperatures. Polycarbonate and poly(ethylene terephthalate) are processed at temperatures above 300.degree. C. A considerable amount of additive may be lost when the hot polymers are exposed to the atmosphere or a vacuum (vented extruders) unless the additive has a very low vapor pressure. Many of the commercial low molecular weight antioxidants are too volatile to be used in these applications.
Polymers are protected against thermooxidative degradation by a variety of antioxidants which are generally classified as either chain breakers or peroxide decomposers. In most cases it is advantageous to have both types of antioxidant present. The chain breakers are stabilizers that intercept the propagation step in the oxidative degradation mechanism and thereby reduce the overall oxidation rate. Hindered phenols, secondary alkylaryl and secondary diarylamines generally fall into this category. The peroxide decomposers decompose hydroperoxides generated during the processing or aging of the polymer, through non-radical reactions thereby inhibiting the chain initiation step. Sulfides such as the well-known dialkyl thiodipropionates, aryl and alkyl phosphites, metal dithiocarbamates and dithiophosphates are among the commonly used peroxide decomposers.
Three factors affect the performance of the stabilizer in a polymer composition. The intrinsic activity of the stabilizer functional group on a molar basis, the compatibility or solubility of the stabilizer in the polymer system, and the ability of the stabilizer to remain in the polymer system. The third factor is often the dominant factor (J. K. Kuczkowski and J. G. Gillick, Rubber Chemistry and Technology, 57, pp 621-651 (1984); G. Scott, New Developments in rubber-Bound Antioxidants, Rubbercon 77, Int. Rubber Conf., 1977, 1, paper #19). Consequently, there has been a considerable amount of effort in the development of stabilizers that are less volatile, more compatible and less readily lost during fabrication and exposure to the environment. Engineering thermoplastics are processed at high temperatures so it is essential to use high molecular weight stabilizers that are not lost through drying, extrusion, and molding steps. For polymers that come in contact with foodstuffs it is important that the stabilizers are non-toxic or cannot be extracted out of the polymer into the foodstuff. Obviously, polymer bound stabilizers are preferred where FDA approval is required in the end-use.
An approach to solving the volatility and migration problems of the stabilizers has been to prepare stabilizers with polymerizable groups and then either polymerize the monomeric stabilizers to homopolymers or copolymerize the stabilizer with the monomer of the polymer requiring stabilization. (J. Fertig, A. L. Goldberg and M. Shoultchi, J. Appl. Polym. Sci., 10, pp 672 (1966); G. Scott, Developments in Polymer Stabilization, Vol 4, G. Scott, Ed., App. Sci. Pub., London, 1981, pp 181).
The more popular approach is to copolymerize the polymerizable stabilizer with another monomer. There are numerous examples of copolymerizable antioxidants found in the literature (J. K. Kuczkowski and J. G. Gillick, Rubber Chemistry and Technology, 57, pp 621-651 (1984)).
Maleimides containing N-substituted stabilizer groups of the prior art have been copolymerized with vinyl monomers to form maleimide copolymers with bound stabilizer groups. U.S. Pat. No. 4,078,091 covers homopolymers and copolymers of N-(3,5-disubstituted-4-hydroxyphenyl)maleimides. U.S. Pat. No. 4,152,319 covers copolymers prepared from the N-(3,5-disubstituted-4-hydroxyphenyl)imides of substituted maleic, itaconic and citraconic anhydrides. Japanese patent 56/139,541 A2 (CA96:70248t) covers copolymers of N-(p-anilinophenyl)-maleimide with acrylonitrile and 1,3-butadiene. Ger. Offen. No. 2,025,336 (CA74:77246y) covers copolymers of the same N-substituted maleimide with isoprene.
Other known copolymerizable N-stabilizer substituted maleimides and the references in which they can be found are as follows: ##STR1## 2,6-di-t-butyl-4-(N-maleimidomethyl)phenol of Canadian Patent No. 677,494; ##STR2## N-(4-(phenylamino)phenyl)maleimide of U.S. Pat. No. 3,767,628; ##STR3## N-[2-hydroxy-3-(3-t-butyl-4-hydroxy-5-methylphenyl)-propyl]maleimide of U.S. Pat. Nos. 3,956,298 and 4,066,616.
Another method of preparing polymer bound stabilizers is to attach stabilizer groups to existing polymers or copolymers. This seems to be a more popular method of preparing polymer bound stabilizers since the stabilizer concentration can be controlled more efficiently and the stabilizer is normally bound directly to the polymer or copolymer.
Antioxidants containing thiol groups were attached to rubber modified thermoplastics such as ABS or other rubber lattices using peroxide initiators. K. W. S. Kularatne and G. Scott, Eur. Polym J., 15, pp 827-32 (1979) and references cited therein).
Borg-Warner patented the method of attaching thiol antioxidants to rubber modified thermoplastics in the presence of a peroxide initiator in a melt processing step (European Patent Application No. 84,882).
A. H. Weinstein incorporated antioxidant groups containing thiol or disulfide groups into polydiene homo- or copolymers by chain transfer methods during the polymerization or by free radical addition to the olefinic units after polymerization. (A. H. Weinstein, Rubber Chemistry and Technology, 50, pp 641, 650 (1977)).
The introduction of antioxidant functions into polydienes during post polymerization interactions of amino, hydroxyl or nitroso substituted antioxidants with epoxy, olefin or carbonyl groups present in the polydiene substrate has been reviewed by Kline and Miller (R. H. Kline, J. P. Miller, Rubber Chemistry and Technology, 46, pp 96-104 (1973)).
N-(2-hydroxyethyl)-S-(2-benzothiazolyl)mercaptoacetamide and N-(2-hydroxyethyl-S-(2-benzimidazolyl)mercaptoacetamide have been reacted with cetyl methacrylate-maleic anhydride copolymer to obtain the corresponding polymer (semiester) bound antioxidants (J. Herdan, L. Crisan, M. Luca, S. Balin, Rev. Roun. Chim., 1983, 28(7), pp 757-762 CA100:86205p). However such semiester linkages are far less thermally stable than the imide linkages of the present invention. Polymeric semiesters are known to be unstable at temperatures used to process thermoplastic compositions and have been used as blowing agents for such compositions (U.S. Pat. No. 4,238,572: Ger. Offen No. 2,757,558:CA89:111460).
There has been a considerable amount of activity in the area of preparing polymer bound stabilizers by modifying copolymers containing reactive functionalities with stabilizers containing groups that react with the reactive functionality of the copolymer. Examples of such modifications include: modification of methacrylic acid/styrene copolymers with stabilizers containing glycidyl groups (Japanese Patents Nos. 69/32,054, 69/32,055 and 69/32,056: CA72: 67742, 67743 and 67744), transesterification of ethylene-vinylacetate with esters of beta-(3,5-di-t-butyl-4-hydroxyphenyl) propionic acid (Japanese Patent No. 76/136,782; CA86; 107573), modification of glycidyl (meth)acrylate copolymers with reactive stabilizers (Japanese Patent No. 69/24,274: CA72: 80236f), modification of vinyl-p-benzyl chloride-polybutadiene copolymers with thiol-containing antioxidants (U.S. Pat. Nos. 4,481,337 and 4,452,939) and modification of carboxy-containing polyolefins with stabilizers containing glycidyl groups (Japanese Patents Nos. 69/02,714, 69/02,715 and 69/02,719: CA70: 107161a, 107162b, 107152y).
Japanese Patent No. 71/26,859 (CA77: 20858d) covers the attachment of antioxidants containing amino groups, hydroxyl groups or isocyanate groups to crosslinked glycidyl methacrylate-divinylbenzene copolymers or crosslinked styrene-maleic anhydride-divinylbenzene copolymers.
None of these prior art references discloses the present invention.